1. Field of the Invention
The present invention relates to a fluorene-based compound and an organic electroluminescent (OEL) display device that uses the same. In particular, the present invention relates to an OtL compound that has a structure that comprises at least one fluorene derivative and at least one carbazole derivative. The present invention also relates to an OEL display device with an organic emission layer or a hole transport layer that includes the OEL compound.
2. Description of the Background
In general, an OEL display device (OEL device) has an anode, a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and a cathode that are sequentially formed on a substrate. The HTL, the EML, and the ETL are organic thin films.
The OEL device with the structure described above operates as follows. When a voltage is applied to the anode and the cathode, holes injected from the anode migrate to the EML via the HTL. Meanwhile, electrons from the cathode are injected in the EML via the ETL. Carriers are recombined in the EML to generate excitons. When these excitons fall from an excited state to a ground state, they emit light in the EML, which forms an image. The light that is emitted when excitons drop to the ground state from a singlet excitation state is referred to as “fluorescence.” The light that is emitted when excitons drop to the ground state from a triplet excitation state is referred to as “phosphorescence.” Fluorescence with a probability of the singlet excitation state of 25% (triplet excitation state: 75%) has a low luminous efficiency, whereas phosphorescence using a singlet excitation state of 25% and a triplet excitation state of 75% can theoretically have an internal quantum efficiency of about 100%.
A green and red high-efficiency OEL device has been developed that uses Ir(ppy)3 and PtOEP which are phosphorescent pigments that have heavy elements Ir and Pt with a high spin-orbital coupling energy at their central positions as dopants. These pigments effectively emit light even in a triple state (phosphorescent) OEL. In this type of device, 4,4′-N,N′-dicarbazole biphenyl (CBP) is used as a host. CBP has a low glass transition temperature of 110° C. or less and is easily crystallized. Due to CBP's unstable properties, this OEL device has a short life span of only 150 hours or less and is thus unsuitable for commercial use.